The present invention relates to a semiconductor integrated circuit device which is mounted on electronic apparatuses, and a method of fabricating the same. More particularly, it relates to techniques that are effective when applied to a semiconductor integrated circuit device which has a microcomputer furnished with a nonvolatile memory circuit.
In order to satisfactorily perform a so-called initial evaluation (debug) such as system check or circuit check at the initial stage of development, a semiconductor integrated circuit device (LSI) having a microcomputer to be mounted on an electronic apparatus should desirably be capable of altering internal data and internal logic with ease. It is therefore a recent trend that an EPROM (Erasable and Programmable Read Only Memory) is built in the semiconductor integrated circuit device as a memory into which microprograms, data programs etc. are written.
The EPROM is a nonvolatile memory circuit into which information is electrically written and from which information is erased with ultraviolet light, so that the information can be rewritten after the production process of the LSI. A technique of this type is described in, for example, the official gazette of Japanese Patent Application Laid-open No. 188234/1984.
More specifically, a semiconductor integrated circuit device which has a built-in EPROM as a memory for writing a program thereinto is used till the stage at which the program for controlling a microcomputer is determined by an initial evaluation.
When the initial evaluation has ended to determine the program for controlling the microcomputer, it becomes unnecessary to use the EPROM as any memory. Since the EPROM has its memory cells formed of field effect transistors of double-layer gate electrode structure, it requires a complicated production process and involves a large number of manufacturing steps. Moreover, the EPROM necessitates windows for ultraviolet erasure, which increase the fabrication cost of a package. For these reasons, the semiconductor integrated circuit device having the microcomputer furnished with the EPROM incurs a high cost of production. Besides, when the articles of the semiconductor integrated circuit device are mass-produced, the determined program needs to be written in the built-in EPROMs of the individual articles, and hence, a period of time for writing information becomes long in the semiconductor integrated circuit device which has the EPROM as a memory.
In order to avoid the above drawbacks, it is practised that, after the program has been determined by the semiconductor integrated circuit device having the microcomputer furnished with the EPROM, a semiconductor integrated circuit device having a microcomputer furnished with a mask ROM is developed anew, whereupon the determined program is set in the built-in mask ROM. The mask ROM is a nonvolatile memory circuit which serves only for reading out information, and into which the information is written in the production process thereof. The mask ROM has a simple construction in which memory cells are formed of field effect transistors of single-layer gate electrode structure, and it is fabricated by a simple production process and a small number of manufacturing steps. Moreover, since the mask ROM does not require the windows for the ultraviolet erasure in the case of the EPROM, the fabrication cost of a package can be reduced. Thus, the semiconductor integrated circuit device having the microcomputer furnished with the mask ROM is inexpensive and is suited to mass production, with the result that the cost of an electronic apparatus can be curtailed.